The cause was cancer, her organization, the Green Belt Movement, said. Kenyan news organizations said she had been treated for ovarian cancer in the past year and had been in a hospital for at least a week when she died.

Maathai was a leading environmentalist and feminist as well as a human-rights advocate. She has also worked to encourage nations around the world to work together to reduce the greenhouse gas emissions causing climate change.

During an interview on Michigan Radio's The Environment Report in 2009 (click on audio above), Maathai also called on everyone to help work to solve the global warming problem.

"I think it’s very important to encourage farmers, individual citizens to plant trees. And, I’m very happy to know that in some of your states, tree planting has been embraced as one of the solutions. It’s one of the activities that every one of us citizens can do and feel good about it, and teach kids to do it, because every tree will count. And when there are 7 billion of us, almost, in the whole world, so you can imagine, if every one of us planted a tree and made sure that tree survived – can you imagine the impact?"